1. Field of the Invention
The present invention relates to a pacemaker of the type having a pulse generator for delivering stimulation pulses to a patient's hear and a control unit for controlling the delivery of stimulation pulses from the pulse generator, the control unit having an altering unit for altering the AV-delay from a predetermined first AV-delay value to a predetermined second AV-delay value, and back to the first AV-delay value, and a sensor being provided to measure a parameter related to cardiac output of the patient.
2. Description of the Prior Art
The PV-interval between the occurrence of an intrinsic P-wave and the application of a cardiac stimulation pulse can be different from the AV-interval between consecutive cardiac stimulation pulses to the atrium and the ventricle.
The term AV delay means the interval between the occurrence of a paced or an intrinsic P-wave and the application of a cardiac stimulation pulse to the ventricle of the heart.
It is well known that cardiac output of a human being is dependent on the 20 AV-delay, see e.g. U.S. Pat. Nos. 5,024,222, and 5,487,752, and 5,540,727, and 5,643,327 and 5,891,176. In all these patents, techniques are described for studying the variation of a cardiac performance parameter of the heart as a function of ao AV-delay in order to optimize the cardiac output. Thus U.S. Pat. No. 5,487,752 discloses a cardiac stimulator system including means for monitoring and pacing two chambers of the heart with pulses providing predetermined AV intervals. One of the AV intervals represents a base line value and the system is operated with this AV interval for a relatively long time, e.g. during 20 beats. The AV interval is changed for relatively few beats, e.g. 5 beats, and is then allowed to return to the base line value. A cardiac performance parameter is measured during these times of different AV intervals and from these measurements it is determined whether an improvement in said cardiac performance parameter over the base line value occurs when the AV interval is changed. By performing this procedure for different AV intervals an optimum AV interval can be determined.
FIG. 1 shows experimental data obtained from an animal study and illustrates the dependence of several cardiac performance parameters on the AV delay (or PV delay). The AV delay was altered between 80 and 130 msec, see curve a. 15 changes were performed during a period of 16 minutes while monitoring the sinus rate, curve b, the heart rate variability (HRV), curve c, the mean artery pressure, curve d, the mean carotid flow, curve e, the central venous pressure (CVP), curve f, the right atrial oxygen pressure (pO2), curve g, and oxygen saturation (SO2), curve h. A very pronounced correlation between the AV-delay, curve a, and some of the measured parameters like heart rate variability (HRV), curve c, mean artery pressure, curved, mean carotid flow, curve e, and central venous pressure (CVP), curve f, is noticed. Corresponding variations in the oxygen pressure (pO2), curve g, and oxygen saturation (SO2), curve h, are less pronounced.